What is the meaning of Lands And Grooves?
Rifling is the spiral patterns found inside the barrels of handguns and rifles. The lands are the raised areas between two grooves, while the flat portions are referred to as grooves. The number, depth, shape, twist orientation (right or left), and twist rate of these lands and grooves can all differ.
A smoothbore weapon is one with no rifling in the barrel. Smoothbores range in size from small handguns to massive tank guns and artillery mortars.
Also Read : Brief History of Forensic Firearm Identification
What are the functions of lands and grooves in a firearms?
When a rifling pattern is created within a rifle barrel, creating these grooves leaves “lands,” or high sections, intact between them. As the bullet travels down the barrel, it spins due to the spiral lands and grooves. The bullet comes into touch with the beginning of the land and groove pattern as it passes from the firing chamber into the barrel. Because of the spiral shape of the lands and grooves, the bullet spins in the same direction as the rifling cuts.
How manufacturers generate rifling in Barrels?
Modern gun manufacturers employ one of three primary machining procedures to generate the rifling in barrels:
a.) A broach can be used to cut rifling into the inner surface of a barrel,
b.) a hardened steel button can be used to make rifling,
c.) Or, hammer forging can be used to form rifling.
Cutting metal from the bore produces rifling with fewer lands and grooves than pushing a tungsten carbide die through the bore which produces “button rifling.” Button rifling is substantially shallower than standard cut rifling because the metal in the bore displaces around the die. The barrel blank is hammered around a mandrel with the rifling pattern reversed to generate hammer-forged rifling.
Electrochemical Rifling is a technique of rifling barrels that avoids the machining operations used by the other approaches.
Each manufacturer has a unique cutting pattern, and the direction and degree of tilt of each twist will be consistent for each type of handgun. A.32 calibre Smith and Wesson pistol, for example, has five lands and grooves in a clockwise sequence inside the barrel, which is known as a “right twist.” A.32 calibre Colt, on the other hand, has six lands and grooves in a counterclockwise arrangement, or a “left twist.” This information is critical to the forensic ballistics investigator’s work. The grooves of a bullet mirror the lands in the gun, and vice versa. However, bullets may not always arrive in perfect condition at the crime lab. These are known as the “class characteristics” and “individual characteristics” of the ammunition.
Also Read: Instruments Used in Forensic Ballistics
The categories of rifling characteristics are:
- Caliber (bore diameter),
- Number of land and grooves
- Direction of twist (the cut grooves produce the bullet’s spin which, in turn, improves accuracy and distance).
- Land and groove impression dimensions.
Impact with objects such as bones or buildings can cause them to deform. Sometimes this causes the bullet to shatter into many piece. An investigator can still collect useful information even if the cartridge casing or bullet is in less-than-perfect condition.
Investigators and/or scientists should keep the following information in mind while studying fired bullets and comparing them to known samples:
- Bullet/slug weight
- The number of land and grooves
- Direction of twist
- Width of lands
- Width of grooves
- Bullet diameter.
- Composition of bullet.
- Manufacturer/marketer of bullet/projectile. If applicable, use reference materials such as an ammunition database.
- Detailed description of the bullet.
- Note type and position of cannelures.
- Note any foreign/extraneous markings—shave marks, flared base, etc.
- If possible, compare marks on bullets with tests from a firearm or with other bullets.
- A discipline mainly concerned with determining whether a bullet or cartridge was fired by a particular weapon.
- Crime (exhibit) bullet:
- The bullet found in body or crime scene
- Test under microscope with test bullet from suspected weapon.
Also Read: Projectiles and Cartridges
Determined by –
- Primary and secondary marking.
- The appearance of the wound.
- Class Characteristics
- Different gun manufacturers use different rifling techniques
- These techniques impart the class characteristics of a bullet
- Number of lands and grooves
- Width of lands and grooves
- Depth of lands and grooves -Pitch
- Individual/accidental marking
- Bullet fingerprint
- A cross section of a gun barrel will show small grooves or striations all along the lands and grooves (machine markings)
- These are created when the barrel is rifled. No two gun barrels have the exact same markings.
- Irregularities on the inner surface of barrel due to accidental manufacturing defect’ or metallic fouling( sticking of particles of prior fired bullet.)